It has long been recognized that the physical properties of a bulk metal object are influenced by its grain structure. For this reason, a great deal of effort has been devoted to finding ways to control grain structure in fabricated metal objects, and a great many examples of this can be found in the literature. However, the vast majority of these methods involve control of grain structure in cast objects, or in objects that are first cast and then worked (e.g., forged metal objects). For these methods of fabrication, there is a practical lower limit for grain size, that being on the order of about 1000 nanometers (i.e., 1 micron). Recently, it has been recognized that metal objects with grain size smaller than 1000 nanometers (nm) may exhibit unique properties due to the disproportionate number of atoms near grain boundaries, and due to "quantum effects" (e.g., incomplete band structure) that can only be observed for grains or particles smaller than about 1000 nm. Objects with grain size smaller than 1000 nm have come to be known as nanostructured objects.
Since conventional casting methods give metal objects with relatively large grains, new technologies have been developed to make nanostructured metal objects. The most successful of these methods is the rapid solidification rate (RSR) process, in which a thin stream of molten alloy is poured onto a rapidly spinning cooled wheel so that a very thin (10-50 micron) ribbon is formed. Because the ribbon is so thin, and because it is cast onto a cold surface, the ribbon cools at an extraordinary rate. This limits the growth of the metal grains within the ribbon, usually to a size less than 1000 nm. In addition to limiting the grain size in the ribbon, the RSR process can also produce ribbons in which the grains adopt a preferred grain orientation. This means that the crystallographic axes of the grains are not oriented randomly, but rather have adopted a preferred orientation with respect to some external reference. This feature is very useful because metal objects with preferred grain orientation may exhibit anisotropic physical properties, such as anisotropic electrical conductivity, anisotropic magnetic behavior, etc. In other words the physical properties of the object exhibit a "preferred orientation" because the grains adopt a preferred grain orientation. However, while the RSR process can produce nanostructured metals which exhibit preferred grain orientation, it should be noted that the process is limited to the production of very thin ribbons.
The use of thin nanostructured ribbons produced by the RSR process in the construction of larger objects, primarily permanent magnets, has been explored and is described in a recent review by Kuhrt (Intermetallics, Volume 3, pp 255-263, 1995). In most cases, the ribbons are crushed into relatively large particles (ca. 1 to 10 microns), which are then placed in a mold or die. A preferred grain orientation is established by aligning the particles magnetically. The particles are then compacted, and finally sintered. It should be noted that, in most cases, magnetic alignment is essential for producing an object with preferred grain orientation as the crushed particles are isometric (i.e. having approximately the same width in any direction, such as a sphere) and therefore cannot be aligned by mechanical methods. An exception to this, however, is disclosed in U.S. Pat. No. 5,009,706 to Sakamoto et al., in which a mechanical alignment is described. Notwithstanding the fact that this disclosure discusses grains that are smaller that 1000 nm in size, it is clear that the compact itself is formed from relatively large particles which polycrystalline, and which are between 10 and 1500 microns in size (see column 7, lines 16 through 27).
It should be emphasized that the cases cited by Kuhrt and by Sakamoto et al. all involve the formation of metal compacts from powders which consist of relatively large (i.e., greater that 1 micron) particles that are polycrystalline. In other words, the compacts are formed from large particles, each of which is composed of a large number of grains. The individual particles may themselves be nanostructured (i.e., have grains smaller that 1000 nm), but the particles comprising the compact are large.
A fundamentally different type of compact would be one formed directly from metal particles which are smaller than 1000 nm. Metal particles smaller than 1000 nm, which are referred to as nanosize particles, usually consist of a single grain. Thus, the significant difference between these compacts and the compacts described by Kuhrt and Sakamoto et al. is that the compacts formed from nanosize particles consist primarily of compacted single grains, whereas the compacts described by Kuhrt and by Sakamoto et al. are comprised of large polycrystalline particles. Advantages of making compacts from nanosize particles include improved control of the grain structure in the compact formed from nanosize particles, and the ability to make compacts with unique composition from mixtures of nanosize particles.
The formation of compacts, including metal compacts, directly from nanosize particles has been discussed in a recent review by Dowding et al. (Advances in Powder Metallurgy & Particulate Materials, Volume 5, Metal Powder Industries Federation, 1994). Dowding et al. point out that this approach has not been extensively explored because of the difficulty in producing nanosize powders in adequate quantities. Recent reviews of methods for producing nanosize particles are presented in texts by Klabunde (Free Atoms, Clusters, and Nanoscale Particles, Academic Press, 1994) and by Schmid (Clusters and Colloids, VCH Publishers, 1994). These confirm the fact that large scale (greater than 1 gram) syntheses of nanosize metal particles have not been developed, and that only a few large scale syntheses of nanosize ceramics particles have been developed. In addition, these reviews reveal that all reported syntheses of nanosize metal particles give particles that have an isometric morphology (i.e. approximately the same width in any direction, such as spheroidal particles or equiaxial particles). This is a serious limitation since isometric particles cannot be easily aligned by mechanical methods.
Several methods for fabricating objects from nanosize particles are described in the review by Dowding et al. However, these procedures typically involve processing under conditions that produce high temperatures. As a result, the objects that are fabricated are fully sintered objects and not compacts. This is undesirable because the sintering process typically changes the grain size in a compact.
U.S. Pat. No. 5,147,446 to Pechenik and Piermarini describes the use of a diamond anvil press to produce objects or compacts from nanosize particles. The compacts made by this process are not known to exhibit any significant degree of preferred grain orientation. In addition, the compacts made by Pechenik and Piermarini do not consist of anisometric particles. As used herein and in the appended claims, anisometric particles are particles that are not isometric, such anisometric particles having a morphology that is, for example, platelet-like, needle-like, etc.
In the Pechenik and Piermarini disclosure, the amount of material used to make the compacts is not mentioned. However, it is understood that the amount must be minuscule (probably less than 1 mg) because diamond anvil presses, such as the press used in their example, are typically constructed from gem-quality diamonds which are, by their very nature, quite small. In a sense, this is an advantage since nanosize particles are generally available only in small quantities. However, the use of a diamond anvil press does not permit the fabrication of objects of useful dimensions.
Pechenik and Piermarini further claim that it is not possible to compact nanosize particles at room temperature to produce objects with acceptable densities (i.e. greater than 50% of the theoretical value). The reason for this is said to be the propensity for nanosize particles to agglomerate at ambient temperatures to form large aggregates which do not pack well (see column 1, lines 27 through 52). Therefore, Pechenik and Piermarini found it necessary to conduct their process at cryogenic temperatures in order to obtain objects with suitable densities (i.e. greater than 50% of the theoretical value). However, processing at cryogenic temperatures is relatively expensive and can be hazardous.
U.S. Pat. No. 4,771,022 to Block et al. discloses a process for producing compacts from powders which may consist of nanosize particles. However, the process involves the conversion of the nanosize particles into a different polymorph. Thus, the process does not give a nanostructured compact of particles, but rather gives an object with a grain structure which is not directly related to the morphology of the particles used in its fabrication. Moreover, the compacts made by this process are not known to exhibit any significant degree of grain orientation, nor do they consist of anisometric particles.
U.S. Pat. No. 4,921,666 to Ishii, and U.S. Pat. No. 4,744,943 to Timm both disclose processes for producing objects or compacts from powders which may contain nanosize particles. However, these processes involve processing at high temperatures. As a result, the objects that are fabricated are fully sintered objects and not compacts. This is undesirable because the sintering process typically changes the grain size in a compact. In addition, the objects made by Ishii and by Timm are not known to exhibit preferred grain orientation, nor do they consist of anisometric particles.
In all of the references cited above, the powders utilized contain particles having an isometric morphology, that is, particles which have approximately the same width in any direction (i.e. spheroidal particles or equiaxial particles). This is because the syntheses disclosed in the references yield particles that are spheroidal since this shape minimizes surface energy and therefore has the greatest stability. However, spheroidal particles cannot be easily aligned by mechanical methods to give an object which exhibits a preferred grain orientation. This is a significant limitation since preferred grain orientation can result in improved performance in an object.
This invention has therefore as its purpose to provide a process for the production of metal compacts which exhibit preferred grain orientation, as by powder metallurgical methods, wherein the preferred grain orientation is provided by mechanical means. It is another object of the invention to provide for the fabrication of nanostructured metal compacts which exhibit preferred grain orientation, as by powder metallurgical processes.